Integrated circuits are being used in an ever increasing variety of application (e.g. computers, communications equipment, automobiles, consumer electronics, etc.). Many of the applications require integrated circuits which produce a minimal amount of electromagnetic noise and which consume a minimal amount of power.
It is common for a data processing system or a portion of a data processing system, which is implemented on an integrated circuit, to have address pins which are capable of being coupled to an external address bus, and to have data pins which are capable of being coupled to an external data bus. Some data processing systems also have control pins, chip select pins, and other types of integrated circuit pins which are also capable of being coupled to conductors that are external to the data processing system.
Unfortunately, however, when integrated circuit pins are coupled to external conductors, it is possible for a large number of the conductors to switch from one logic state to another logic state at approximately the same time. For example, the conductors for the external address and data buses may switch from mostly ones to mostly zeros at approximately the same time.
This switching of the external bus conductors from one voltage level to another voltage level produces a significant amount of electromagnetic interference (EMI) emissions, which are generally referred to as noise. In some applications, such as automobile electronics and communications electronics, the electromagnetic interference can be a significant problem. In fact, many applications using integrated circuits require the noise produced by EMI emissions to be reduced as much as possible.
Because many integrated circuits today operate in the megahertz frequency range, the noise produced by EMI emissions from many integrated circuits is in the radio frequency range. Noise in the radio frequency range is especially problematic when the application using the integrated circuit involves communications, such as radios, pagers, cellular phones, etc.
In addition, the capacitive charging and discharging of the external bus conductors may consume a significant amount of power. And reducing the power consumed by integrated circuits is becoming very important as more and more products become portable and use a battery as a power source (e.g. portable computers, pagers, portable telephones).
Some prior art integrated circuits tri-state their outputs (i.e. place the outputs in a high-impedance state) in order to reduce noise and power consumption when the corresponding external conductors are not being used. When an output is tri-stated, the voltage level on its corresponding external conductor is indeterminate. Unfortunately, however, some peripheral integrated circuits will not function properly if they are coupled to tri-stated outputs.
In order to accommodate peripherals which cannot handle tri-stated inputs, some integrated circuits drive all of their outputs to a predetermined voltage level when the corresponding external conductors are not being used. For example, some integrated circuits drive all of their outputs to the voltage level corresponding to a logic level one when the external conductors are not being used. Unfortunately, however, this approach produces more bus switching than the tri-state approach, and thus produces more noise and consumes more power.